Various techniques are known for establishing a rotating field in an AC motor by providing phase shifted fields in separate windings. Numerous systems are known for providing the phase shift, including various combinations of resistors, capacitors or inductors to store electrical energy, and then release it as phase shifted alternating current.
Various AC frequency conversion techniques are likewise known in the art, including the use of an H-switch for such purpose. In one type of single phase cycloconverter, an H-switch connects a motor winding to a pair of power lines L1 and L2 supplied by an AC source. The H-switch has one ON state connecting L1 to the left end of the motor winding and connecting L2 to the right end of the motor winding. The H-switch has another ON state connecting L2 to the left end of the motor winding and connecting L1 to the right end of the winding. The H-switch is switched between its two ON states at a given periodic frequency which mixes with the frequency of the input AC signal in heterodyning relation to yield various output frequencies, analogous to side bands. For example, if a 60 hertz input AC signal is mixed with a switching rate of 132 hertz for the H-switch, a plurality of side bands result, including 72 hertz, 192 hertz, 336 hertz, etc., Static Power Frequency Changers, Gyugyi and Pelly, John Wiley & Sons, N.Y., 1976. In the classical cycloconverter approach, during a given half cycle of the output frequency, the output waveform may be constituted by a plurality of segments of both positive and negative polarity, for example pages 56, 57 and 168 of the noted Gyugyi reference.
The present invention provides a particularly simple technique for using H-switches to provide the requisite phase shift for starting an AC motor in combination with the provision of a frequency converted run mode after start-up. In the start-up mode, one or more H-switches are used to provide the desired phase shift between first and second motor windings for motor start-up, without energy-storage capacitors or inductors. At the termination of the start-up mode, the first motor winding is disconnected from the AC source, and an irregular timing pattern is used to toggle the H-switch for the second winding between its two ON states. This irregular timing of H-switch switching yields a chopped sinusoid output waveform of a given fundamental frequency. In preferred form, the H-switch is toggled in response to a given frequency clock signal and is also toggled in response to each zero crossing of the input AC signal except when a zero crossing and a clock signal coincide in time. In contrast to the classical cycloconverter approach, in the present invention the output waveform in any half cycle during the frequency converted run mode is constituted by segments of common polarity.
The present invention provides in preferred form a start-up control and a frequency conversion combination with a simple logic system coordinating the requisite timing pattern for switching one or more H-switches and their respective motor windings between one ON state and an OFF state in a given sequence during start-up to afford the requisite phase shift and a rotating field, and providing the requisite irregular timing pattern to switch one of the H-switches and its motor winding between one and another ON state to yield a frequency converted chopped sinusoid output waveform. The coordinating logic system combination provides a high degree of flexibility with a minimum number of parts, including common usage of parts in each mode.